Nanoindentation of Atomically Modified Surfaces
- PDF / 427,821 Bytes
- 8 Pages / 414.72 x 648 pts Page_size
- 52 Downloads / 266 Views
properties of small volumes is often found to be different than that of the bulk. In fact, Gane and Bowden [I] were the first to note that a metals resistance to indentation on a small scale was quite different than the conventional microhardness measurements. They observed that a critical load must be reached before a stylus could penetrate into the surface of a Au crystal. The critical load approached that of the theoretical shear strength of the crystal although the authors report quite a variability in the load which most likely resulted from organic impurities adsorbed on the Au surface. Previously, we investigated the yield behavior of well prepared single crystal Au surfaces which were free of any surface oxides and contamination layers [2]. On these surfaces, we found that the yield point also approached the theoretical shear strength of gold. Beyond the yield point, the deformation was composed of discrete plastic events followed by elastic loading. This behavior was interpreted as multiple dislocation nucleation and multiplication events occurring under the indenter tip and was modeled following the analysis of Gerberich et. al. [3]. We also observed an increase in hardness, as shown in Fig. 1, if we allowed the surface to contaminate with organics from the ambient. Since we only expect the thickness of the organic contamination layer to be on the order of I nm, it is difficult to rationalize the increase in hardness observed from a simple analysis of the mechanical properties of a thin overlayer. We thus speculated that the contamination might effect the energetics of dislocation nucleation under an indenter tip by changing the value of the surface thermodynamic properties (surface energy and surface stress [4]). This paper is aimed at investigating the effect on nanoindentation of 77 Mat. Res. Soc. Symp. Proc. Vol. 505 © 1998 Materials Research Society
changing the surface thermodynamic properties in a controlled way via electrochemical modification. 50
50
40
40 .30
Z 30
010
0
5
10
15
20
25
0
30
5
10
15
20
25
30
Indentation depth, nm
Indentation depth, nm
FIG 1. (a) Load-displacement data for three indentations on the clean Au (11) surface. (b) Load-displacement data for three indentations on the contaminated Au (I 11) surface. Notice the irreproducibility in (b) as compared to (a) as well as the decreased indentation depths (increased hardness) in (b). Electrochemistry gives us a unique opportunity to modify the surface of metals in a controlled manner. In this paper we will be looking at three types of modifications. The first involves the deposition of a single monolayer of another metal on the Au surface by a process known as underpotential deposition [5]. We can tailor the amount of strain in the overlayer by our choice of deposited metal. Ag for example has less than a 0.02 % lattice misfit with Au while that for Pb has a over a 17 % misfit. The second modification involves oxidizing the Au surface with a monolayer of Au 20. The third modification is to reconstruct the Au s
Data Loading...
